1. Field of the Invention
This invention relates to a density meter which is inserted directly into a muddy liquid to measure the degree of opacity thereof, and more particularly to a density meter which is maintained in the muddy liquid for the calibration thereof.
2. Description of the Prior Art
In a measurement device utilizing optical means for measuring the density of a liquid, a muddly liquid is irradiated by a light emitting unit provided on one side of a container sampling the muddy liquid. Light passed through the muddy liquid and/or light scattered by suspended solids in the muddy liquid is received by a light receiving unit, and the output signal level of the light receiving unit is utilized to measure the density of the muddy liquid.
In a measurement device of the above type, which is inserted directly into a muddy liquid to measure the density thereof, it is unnecessary to transfer the muddy liquid for the sampling thereof. Accordingly, the measurement device is free from the problem that the pipes become clogged up with mud, and it does not need a large installation space. Furthermore, the measurement device is small in size and light in weight, and therefore it can be readily handled. In addition, the measurement device is automatically cleaned by a seal member which is provided at the end of a piston adapted to introduce the muddy liquid into the container, which contributes to improvement of the measurement accuracy and the reliability. Moreover, as the reliability of its consumable components (such as for instance the light emitting unit) is improved, it is unnecessary to replace these components for at least one year.
Although the conventional density meter is advantageous as described above, it still involves a problem of calibration. With the conventional density meter, before it is set in a muddy liquid to be measured, calibration is carried out by putting a transparent liquid (pure water), a standard specimen (formazine or kaolin) or a liquid having a known density in a transparent inner cylinder of the meter. After the calibration has been achieved, the densitometer is set in the muddy liquid to start the measurement of the density of the muddy liquid.
However, after the density meter has been used for a long period, it is impossible for the density meter to accurately measure the density of a liquid with the value which has been set initially, because of vibrations in characteristics of the components, deterioration of the components, and variations of the environmental conditions. Therefore, it is necessary to take the density meter out of the muddy liquid to recalibrate it at suitable time intervals (at least once a month). The density meter thus recalibrated is set in the muddy liquid again. However, such calibration takes much more time and labor than the replacement of the components (such as for instance the light emitting unit). Thus, even if the reliability of the density meter has been improved, it cannot be said that it is free of maintenance.